Male snakes favour manhood over dinner

Snakes on a small Taiwanese island would rather abandon a food source than risk losing their twin penis, according to new research.

In their study of the kukrisnake (Oligodon formosanus), published in Proceedings of the National Academy of Sciences, the team of Australian and Taiwanese researchers claim to have documented for the first time a case of snakes being territorial.

"There are a couple of thousand species of snakes out there and nobody has ever described territorial defence in any snake," says co-author of the study, Dr Rick Shine of the University of Sydney.

"It's very common in lizards," he says, "but the kind of things the snakes eat are not the kind of things that you can defend."

He explains that because snakes are predatory carnivores, they rely on mobile food sources. That means territoriality usually isn't viable.

"If a viper sits there beside a rodent path waiting for a mouse to come past, that mouse is a single meal and once it's been eaten it's all over," he says.

"The thing about a sea turtle nest is that it's a huge amount of food relative to the size of the snake; and it's edible for quite a long time," says Dr Shine.

They observed that male snakes usually found the nests first; but then females would arrive and turf them out. However, if a second female arrived, after an initial combat they would often share the resource.

Why they behave like this comes down to a combination of dentition (they aren't called the 'kukri' snake for nothing), aggressive-defensive behaviour, and the male's sex organs.

"The kukrisnakes, with these very large blade-like teeth, make a huge slashing wound. It's a really nasty bite," says Dr Shine.

"[They] also have a defence display where they lift the back part of their body and they wave their tail around."

Dr Shine says this behaviour is quite common in snakes and is designed to confuse birds or other predatory animals.

"The male [kukrisnakes] actually take it further and evert their hemipenis (twin penis) and wave them around," he says.

"This is a bad idea if there's an aggressive snake with very large teeth, that's going to slash away at the first thing you poke towards her. A good bite in that part of their anatomy, and their evolutionary fitness has probably come to an end."

So when confronted, the male snakes abandon the eggs rather than risk cutting short their reproductivity.

"This is a spectacular example of how little we know about the private lives of animals and the way evolutionary processes can throw up exceptions to almost any rule that we come up with," says Dr Shine.